1. Field of the Invention
The present invention relates to a variable valve control apparatus capable of changing the timing of opening/closing and the lifting stroke of at least one of the intake valve and the exhaust valve of an internal combustion engine (as will be shortly referred to as the "engine") in accordance with the running conditions.
2. Description of Related Art
As disclosed in JP-A-9-32519, there is a known variable valve control apparatus in which the valve-open period and the lifting stroke of at least one of an intake valve and an exhaust valve are changed by axially shifting a camshaft including a cam having an axially different profile.
Another variable valve control apparatus in which the rotational phase of a camshaft with respect to a crankshaft is adjusted to adjust the value opening/closing timing variably is disclosed in JP-A-1-92504.
Another variable valve control apparatus in which camshafts for driving the intake valve and the exhaust valve respectively are coupled by gears and in which the camshaft for receiving the torque of the crankshaft drives the other camshaft is disclosed in JP-A-5-106411.
When axial moving means for changing the valve-open period and the lifting stroke of at least one of the aforementioned intake valve and exhaust valve and phase adjusting means for adjusting the rotational phase of the camshaft with respect to the crankshaft are combined, a separate arrangement of the two means increases the number of parts and accordingly the number of their assembling steps.
It may be possible to combine the camshafts through gears such that one camshaft drives the other camshaft. When one of the camshafts axially moves, however, a gear with the moved camshaft also moves together with the camshaft, so that the gear engagement may disappear or their coupling length may be shortened to fail to transmit the torque sufficiently. This torque transmission could be retained by enlarging the axial length of the gears, but such enlargement of the gears may cause a problem that the gears and the entire apparatus will be increased in size.
Since the torque necessary for driving the cam-shafts is high, helical gears are generally used for transmitting the torque between the camshafts. With this helical gear coupling, when one camshaft axially moves, the other camshaft rotates relative to the one camshaft, so that the relative phase between these camshafts may be changed.
If the gear coupling is applied to spur gears, one camshaft does not change in phase even when the other camshaft moves in the axial direction. If the phase of one camshaft changes relative to the crankshaft, however, the other camshaft also changes in phase, so that the relative phase between these camshafts cannot be adjusted.